Machine for wave soldering of tinning

ABSTRACT

The invention relates to a machine for wave soldering or tinning comprising a solder reservoir ( 9 ); means that can form at least one solder wave with a so-called laminar form ( 8   b ); a conveyor system ( 2 ) that can bring a piece ( 1 ) to be soldered or tinned into contact with said laminar wave; and means for injecting ( 12 ) a gas in the vicinity of the wave; characterized in that the said injection means includes an injector located in a position adjacent to and downstream from the wave and provided with a wall ( 17 ) facing the solder wave, wherein the wall of the injector has at least a first group ( 15 ) of openings positioned so as to provide a first gas jet directed toward the flat surface of the solder wave.

This application is a divisional of Ser. No. 09/514,950, filed Feb. 28,2000, now U.S. Pat. No. 6,223,969, which is a divisional of Ser. No.08/968,448, filed Nov. 12, 1997, now U.S. Pat. No. 6,082,602, which is adivisional of Ser. No. 08/659,042, filed Jun. 3, 1996, now U.S. Pat. No.5,725,143.

FIELD OF THE INVENTION

The invention relates to soldering and tinning operations carried outusing machines comprising a bath of liquid solder, wherein the bath iseither a “static bath” or is set in motion as in the machines known as“wave soldering” machines.

These machines are used in particular for soldering electroniccomponents on a support such as an electronic circuit, or for tinningthe terminals of electronic components.

Description of the Related Art

Wave soldering machines are designed in such a manner that the circuitsto be soldered (or the pieces to be tinned) are brought into contactwith one or more waves of liquid solder generated by pumping a solderbath residing in a tank through a nozzle.

The pieces are generally fluxed beforehand in a zone upstream from themachine, primarily to deoxidize the metal surfaces in order tofacilitate their subsequent wetting by the solder. The fluxing operationis followed by a preheating operation which is carried out both in orderto activate the flux previously deposited on the circuit and in order topreheat the circuits and components prior to their arrival in the hotsoldering zone.

The geometric configuration of the nozzle determines the shape of thesolder wave. Wave soldering machines usually have two waves, a firstso-called “turbulent” wave and a second so-called “laminar” wave thatpresents a relatively large flat upper surface.

In the absence of pieces to be soldered or tinned in the machine, theliquid solder in this laminar wave area flows at very low speed in theupstream direction of the machine. When a piece arrives in contact withthe laminar wave, a partial reversal of the alloy flow occurs and partof the alloy flows in the downstream direction of the machine.

The machines are therefore usually provided with what can be describedas a weir system, whose height can be used to control the flow rate ofthe downstream flow of the solder. This weir system can consist simplyof a metal plate or a guide chute for returning the solder to thesurrounding bath.

One notes that the flow rate and the direction of flow of the alloy inthis laminar wave area have a determining influence on the quality ofthe resulting soldering.

It must also be noted that some users, in order to adapt to the veryspecific characteristics of their production, substantially limit thisdownstream solder flow, preferring to establish a very slight or nearlyzero downstream overflow of solder.

Wave soldering (or tinning) machines are usually open to the ambient airatmosphere. One problem encountered by the users of such machines is theformation of oxide layers (called dross) at the surface of the solderbath as a result of its exposure to air, resulting in a notinsubstantial loss of solder and the need to regularly clean the bath.For example, a medium-size machine can give rise to the formation ofmore than a kilogram of dross per hour of operation.

Considering the specific case of the laminar wave, it is readilyunderstood that a zero or excessively small downstream overflow ofsolder will represent a major disadvantage since the dross constantlyforming on the flat surface of the wave cannot be effectively eliminatedand thus is deposited on the piece with significant adverse effects onthe quality of the resulting soldering or tinning.

Without requiring any further description, it will be readilycomprehended that this dross-formation phenomenon, described here atlength for the case of the flat surface of the laminar wave in a wavesoldering machine, applies even more to the flat surface of a staticbath.

Various technical solutions have been heretofore proposed for protectingthe solder bath from oxidation by the surrounding air. These solutionscan be schematically divided into the following three categories.

a) A first category of solutions consists of setting up a confinedprotection atmosphere, at least above the solder bath, but alsosometimes in the rest of the machine. Thus, completely inertizedmachines have appeared, designed from the outset as a gas-tight tunnel,but there have also appeared more simply cowling or hood systems thatcan be used on existing conventional machines open to the ambient air toset up a nitrogen blanket at least in the area of the solder bath.

In this first category of solutions, the applicant of U.S. Pat. No.5,161,727 has proposed an inertization apparatus comprising a set ofcowlings defining, at least directly above the solder bath, an interiorspace separated from the surrounding atmosphere by gas-tight means, withsystems of gas injection channels opening into the upper parts of thediffuser-equipped cowlings.

While the inertization apparatus described in the aforesaid documentcertainly represents a substantial improvement over the performance ofthe existing systems (particularly in terms of optimizing the compromisebetween the flow rate of the injected gas and the concentration ofresidual oxygen achieved above the solder bath), this system stillrepresents relatively complicated and costly designs since it must bevirtually custom-fitted to each type of wave soldering machine presenton the market.

b) A second category of solutions involves setting up an unconfinedprotective atmosphere using injectors localized in proximity to thesolder wave without closure of the space above the waves.

The devices taught in WO 93/11653 fall into this second category.

Taking into account their extremely localized configuration, it isconsidered difficult to control the quality of inertization afforded bythese processes, necessitating in practice the use of two symmetricalinjectors to successfully achieve a low oxygen level.

Furthermore, none of the documents in this category of solutions dealwith and provide solutions for the specific problems posed by laminarwaves.

c) The third category of solutions to the problem of dross formationemploys the use, at the surface of the laminar wave, of a film of oilwith a high covering power.

The oil protection systems have the traditional disadvantages associatedwith the use of oil (particularly in the presence of a heat source),which in particular include the presence of oil deposits on the board(necessitating the implementation of an often difficult and imperfectcleaning), the necessity for scheduling frequent periods for machinemaintenance due to the accumulation of oil in the solder bath, and oilvapor emissions which certainly represent a nuisance for theenvironment, whether for people or equipment.

SUMMARY AND OBJECTS OF THE INVENTION

One object of the present invention is therefore to propose a wavesoldering or tinning machine that can achieve a localized inertization(without requiring the use of a confinement system) of the flat surfaceof a laminar wave (as already indicated, the laminar wave exhibits veryspecific operating problems due to its extremely low flow) wherein thedesign of the machine makes it possible to very simply and economicallyobtain a very favorable compromise between the residual oxygen contentachieved at the surface of the laminar wave and the flow rate of the gasemployed in the area of this wave, wherein this flow rate, whennecessary to conform to the economic specifications of particular usersites, can be below 10 m³/h and is preferably less than or equal to 5m³/h.

Another object of the present invention is to propose conditions whichmake it possible to achieve a significant reduction in dross formationon the flat surface of the laminar wave of the machine.

Studies conducted by the applicant have shown that such results can beobtained by the use of a gas injector that is localized in a positionadjacent to and downstream from the laminar wave and is provided with awall facing this wave wherein this wall has at least one group ofopenings positioned thereon so as to generate a first gas jet directedtoward the flat surface of the laminar wave.

These studies also demonstrated the advantageous features of thecombined use of the following measures:

use of a weir system having, for example, the form of a plate or a guidechute, for the downstream spillover of the solder into the bath, whereinadjustment of the height of the weir system with respect to the wavepermits adjustment of the flow rate of the laminar wave overflow in theforward direction (i.e., the downstream direction of the machine);

in the case of use of a plate, having this plate dip into the solderbath;

in the case of use of a chute, using a chute which dips into the bath,or equipping the chute with a skirt which dips into the bath, in orderto extend its action to some degree; and

use of a gas injector located in a position adjacent to and downstreamfrom the laminar wave and provided with a wall facing this wave, whereinthis wall has at least two groups of openings that direct the gas towardthe wave, with a first group of openings being positioned so as togenerate a first gas jet directed toward the flat surface of the laminarwave, while a second group of openings is positioned on the wall so asto inject a second gas jet into the space located between the plate andthe injector (in the case of a “plate” weir) or in the interior of theskirt (in the case of “chute” weir).

As developed in greater detail below in connection with examples, thecombined use of these measures gave an extremely effective inertization,even in the difficult case of a very nearly stagnant laminar wave,without requiring any confinement means: it was thus demonstrated that,at the time of board passage, a residual oxygen content of only a fewtens of ppm could be obtained using a very moderate gas delivery rate(of only a few m³/h).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a conventional structure for awave soldering machine;

FIG. 2 is a schematic partial section of a structure having two waves,turbulent and laminar;

FIG. 3 is a schematic representation of a laminar wave before thearrival of a piece (flow of solder in the upstream direction);

FIG. 4 is a schematic representation of a laminar wave during soldering(with a partially reversed solder flow: a part of the solder flows inthe downstream direction with spillover into the chute 10);

FIG. 5 is a partial schematic representation of a machine according tothe invention;

FIG. 6 is a partial schematic representation of a machine according tothe invention, in which the injection means incorporates a deflectingpiece 13;

FIG. 7 is a detailed view of the injector 12 of FIG. 6 incorporating adeflecting piece 13;

FIG. 8 is a schematic representation of one example of implementation ofthe chute/injector assembly when they have a common wall; and

FIGS. 9, 10 and 11 schematically show three machine structures that wereused to exemplify the implementation of the invention as well ascomparative examples (examples which will be described in detail belowin the present description).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The wave soldering or tinning machine according to the invention thencomprises:

a solder reservoir;

means that forms at least one solder wave having a so-called laminarform;

a conveyor system that brings a piece to be soldered or tinned intocontact with the laminar wave; and

means for injecting gas in the vicinity of the wave; and ischaracterized in that the injection means includes an injector locatedin a position adjacent to and downstream from the wave and provided witha wall facing the solder wave, wherein this wall has at least a firstgroup of openings positioned so as to generate a first gas jet directedtoward the flat surface of the solder wave.

As indicated previously, according to an advantageous mode ofimplementation of the invention, the machine comprises a weir systemwhose height adjustment with respect to the wave serves to control theoverflow flow rate of the wave in the downstream direction, wherein thesaid weir system takes the form of a chute that dips into the solderreservoir or is provided with a skirt structure that dips into thesolder reservoir and the wall has a second group of openings positionedthereon so as to inject a second gas jet into the interior of the chuteor the skirt.

According to one embodiment of the invention, the injector and the chutethen have a common wall, which is provided with injection openings. Aswill be shown more clearly later in connection with the figures, withsuch a configuration the gas of the second jet passes directly from theinterior of the injector to the interior of the skirt.

This configuration offers significant advantages in terms of ease offabrication of the assembly and thus in the cost of implementation.

The injection of a second gas jet into the interior of the skirt can,for example, be implemented by arranging the second group of openings toface an opening whose boundaries lie in the wall of the skirt, or at theinterface between the skirt and the chute, or in the lower part of thechute, depending on the initial geometry adopted.

According to an alternative embodiment of the invention, the machinecomprises a weir system that takes the form of a plate that dips intothe solder reservoir, wherein the wall of the injector then has a secondgroup of openings positioned on the wall so as to inject a second gasjet into the space located between the plate and the injector. It isunderstood that in this last configuration the “chute” is here formed insome manner between the plate and the injector.

The “downstream” direction is to be understood as the direction in whichthe pieces to be soldered are conveyed in the machine.

The gas delivery “openings” according to the invention are to beunderstood as any type of hole configuration that allows the gas toescape from the injector, as is the case, for example, with conventionalcircular openings or slits.

The first group of openings is advantageously positioned on the wall ofthe injector so as to direct the first gas jet tangential to the flatsurface of the laminar wave. Such an arrangement will help produce aCoanda effect for the gas jet on the surface of the wave, thus renderingthe inertization more effective.

The described arrangement for injection of the second gas jet into theinterior of the skirt proves unexpectedly effective for avoiding theparasitic phenomena of air entrainment by the first gas jet.

The openings of at least one of the groups of openings areadvantageously dimensioned so as to obtain at the outlet of the openingsunder consideration a gas velocity in the interval between 0.5 and 30m/s. Preferably, however, the gas velocity will be in a velocity rangefrom 0.5 to 10 m/s, and more preferably in a narrower range from 0.5 to5 m/s.

According to one embodiment of the invention, the injection meansextends over the entire width of the laminar wave, thus more effectivelyaccommodating the dimensions of all the pieces that can be treated inthe machine under consideration.

According to one embodiment of the invention, the first group ofopenings consists of at least one row of slits whose boundaries lie inthe wall of the injector facing the wave.

The injection means can comprise means for delivering the gas to theinterior of the injector, which can, for example, consist of a poroustube located well inside the interior of the injector and adapted forfeed from the outside with the gas under consideration, or which canconsist of a simple “clarinet,” a term that has been traditionallyapplied to refer to a tube pierced with holes.

According to one aspect of the invention, the injector is provided withan upper face or wall. The studies conducted by the applicant havedemonstrated that it is then advantageous to employ a deflecting piecesolidly attached to this upper wall.

Such a deflecting piece preferably extends along the entire length ofthe injector.

According to an advantageous embodiment of the invention, the deflectingpiece (as necessary taking into account the position of the injectorwith respect to the wave) is dimensioned in its width so as to at leastcover the weir system (for example, the chute).

The position of the injector will preferably be adjustable horizontallyand/or vertically.

As will be clearly apparent to the individual skilled in the art, thesoldering or tinning machine according to the invention permits use inthe injector of any type of gas, whether it be a neutral gas (such asnitrogen, regardless of its mode of production and its purity) or a moreactive gas such as, for example, neutral gas/reducing gas mixtures.

The invention also concerns a method for the wave soldering or tinningof a piece in which a piece to be soldered or tinned is brought intocontact with at least one wave of liquid solder in so-called laminarform and in which a protective gas is directed over at least a portionof the wave, wherein said method is characterized in that the protectivegas is directed over the wave by means of an injector that is located ina position adjacent to and downstream from the wave and is provided witha wall facing the wave, wherein at least one group of openings ispositioned on the wall so as to generate a first gas jet directed towardthe flat surface of the laminar wave and the velocity of the gas as itemerges from the openings is between 0.5 and 30 m/s, but preferablybetween 0.5 and 10 m/s, and more preferably between 0.5 and 5 m/s.

According to an advantageous embodiment of the method according to theinvention, the flow rate of the solder wave overflow in the downstreamdirection is controlled by the use of a weir system whose height isadjustable with respect to the wave, wherein the said weir system hasthe form of a chute that dips into the solder reservoir or is providedwith a skirt that dips into the solder reservoir and the wall of theinjector then has a second group of openings positioned thereon so as toinject a second gas jet into the interior of the descending chute orskirt.

In an alternative embodiment, the flow rate of solder wave overflow inthe downstream direction is adjusted through the use of a weir systemthat has the form of a plate that dips into the solder reservoir,wherein the wall of the injector has a second group of openingspositioned thereon so as to inject a second gas jet into the spacelocated between the plate and the injector.

The invention also relates to a device for inertizing the laminar waveof a wave soldering or tinning machine, that has a gas injector locatedin a position adjacent to and downstream from the laminar wave andprovided with a wall facing the laminar wave, wherein the said wall hasat least a first group of openings positioned thereon so as to generatea first gas jet directed toward the flat surface of the laminar wave.

According to one implementation of the invention, the injector isprovided with an upper wall and the device includes a deflecting piecesolidly attached to the upper wall of the injector.

According to one aspect of the invention, the soldering or tinningmachine comprises a weir system whose height adjustment with respect tothe wave serves to control the flow rate of solder wave overflow in thedownstream direction of the machine, wherein the weir system has theform of a plate dipping into the solder reservoir or a guide chute thatdips into the solder reservoir or is provided with a skirt that dipsinto the solder reservoir and the wall of the injector of the devicethen has a second group of openings positioned on the wall so as togenerate a second gas jet directed at the interior of the chute or atthe interior of the space between the plate and the injector.

The wave soldering machine shown schematically in FIG. 1 comprises threezones: a zone I for fluxing pieces 1 with a fluxing system 3 (forexample, a wet type); a zone II for preheating the fluxed pieces thatuses a means 4, for example, consisting of infrared lamps; and a zoneIII for soldering proper, where the pieces 1 encounter, in the mode ofimplementation shown, a single solder wave 8 obtained by pumping (7) thesolder bath 9 through a solder nozzle 6.

The boards 1 are conveyed through the different zones of the machine bya conveyor system 2, for example, consisting of frames traveling alongtwo side belts located on each side of the machine, or “finger”-typeconveyor chains.

FIG. 2 provides a schematic partial view, in section, of a case in whichthe solder bath 9 is used to produce a dual-wave structure, with a firstso-called turbulent wave 8 a having a relatively steep structure(obtained as a result of the structure of nozzle 6 a), and a second wave8 b of laminar structure that offers a flat upper surface of relativelylarge size and is obtained as a result of the structure of nozzle 6 b.

FIGS. 3 and 4 illustrate the solder flow in the laminar wave 8 b,respectively, prior to the arrival of a piece and during the solderingof a board 1.

FIG. 3 thus illustrates the state prior to the arrival of a piece withflow of the solder in the upstream direction of the machine. Theembodiment represented here includes the use of a weir system 10, whichin this case takes the form of a guide chute that is located justdownstream from the nozzle. Adjusting the height of the guide chutefunctions to control the flow rate of solder spillover in the downstreamdirection (here, a zero or nearly zero flow under the givencircumstances).

Although this figure shows a vertical section of the chute structure, aswill be clearly apparent to the individual skilled in the art, the spacebetween the two “walls” of the chute is closed at its two ends by thesides of the reservoir or by two special plates arranged at each end(defining in horizontal section a rectangular or quasirectangularstructure).

The possible phenomenon of partial downstream spillover is bettervisualized in the context of FIG. 4.

The arrival of the piece 1 on the laminar wave causes a partial reversalof the flow of the liquid solder into the downstream direction of themachine (i.e., in the forward direction). The spillover flow rate in theforward direction can be controlled by the height of the chute system10. In addition, the use of such a chute (in place of a simple platejoined to the nozzle 6 b) affords a better guidance and return of thesolder spillover into the bath 9.

FIG. 5 is a partial schematic illustration of one mode of implementationof a wave soldering or tinning machine according to the invention. Thisrepresentation is a partial representation in that it focuses on thelaminar wave/injector/chute/skirt arrangement.

The wave here is represented in the position prior to the arrival of apiece with flow in the upstream direction.

The figure depicts an immersed skirt 11 that is solidly attached to thechute system 10, facing which is a gas injector 12 having one face orwall 17 that has two groups of openings 15 and 16 whose structure willbe described in detail later in connection with the perspective view inFIG. 7.

As will be understood in light of the entire preceding description, wehave chosen to represent in FIG. 5 the chute and skirt solidly attachedthereto by two different types of lines in order to make it easier tounderstand the figure. As already indicated earlier, the chute and skirtaccording to the invention need not be two disjointed pieces that havebeen solidly attached, but rather one can use an immersed chute from thebeginning.

The groups of openings 15 and 16 are respectively positioned so as todirect a first gas jet toward the flat surface of the laminar wave 8 band a second gas jet into the interior of the immersed skirt 11. Asindicated above, the presence of the immersed skirt 11 and the secondgas jet in the interior of the skirt is especially effective foravoiding any effects from air entrainment on the flat surface of thelaminar wave.

The interior of the injector 12 contains a porous tube 14, which issupplied from the outside with gas and which distributes this gas in theinterior of the expansion chamber that constitutes the body of theinjector 12.

The structure shown schematically in FIG. 6 is similar to thatrepresented in FIG. 5, with the exception of the presence of adeflecting piece 13 solidly attached to an upper wall 19 of the injector12.

Here also, as developed in greater detail below in connection withexamples, the presence of the deflecting piece 13 proves to be veryeffective for achieving—if necessary—very low residual oxygen levels inthe area of the flat surface of the laminar wave.

This FIG. 6 illustrates an advantageous embodiment of the inventionwherein the width of the deflecting piece (if need be, in conjunctionwith the position of the injector with respect to the wave) permits itto extend far enough to at least cover the chute system.

FIG. 7, which is a detailed partial view of the injector 12 and thedeflecting piece 13 in FIG. 6, allows a better comprehension of therespective positions of the deflecting piece 13 and the injector 12—inparticular the disposition of this deflecting piece 13 on the upper wall19 of the injector. This figure also provides a better visualizaton ofthe configuration of the openings of the two groups of openings 15 and16.

In the embodiment represented, the groups of openings 15 and 16 arecomposed, respectively, of three rows of slits 18 and of one row ofslits 18, in each case located in the wall 17 of the injector 12 andextending along the entire length of this injector.

FIG. 8 illustrates a particular mode of the invention in which the chuteand injector have a common wall. One notes in effect that the wall 17 ofthe injector here forms one of the walls of the chute, wherein this wallis extended as one of the sides of the skirt 11.

One also notes that in such a configuration the second gas jetoriginating with the second group 16 of openings passes directly fromthe interior of the injector 12 to the interior of the skirt 11.

FIGS. 9, 10, and 11 illustrate configurations that implement theexamples detailed below:

FIG. 9: the chute 10 is not provided with a skirt (or more simply thechute is not an immersed chute) and the wall 17 of the injector is notprovided with the second group of openings;

FIG. 10: the chute 10 is provided with a skirt 11 and the wall 17 of theinjector is provided with the second group of openings; and

FIG. 11: a configuration identical to that of FIG. 10, but includingalso a deflecting piece 13.

A first set of evaluations was carried out using these threeconfigurations in a dual-wave soldering machine: the residual oxygenlevel at the surface of the laminar wave was measured both in thepresence and absence of boards to be soldered. The formation of dross,or absence thereof, on the flat surface of the laminar wave was alsosystematically evaluated.

In all cases, the nitrogen delivery rate employed by the injector wasapproximately 80 L/minute.

The nitrogen used was cryogenic nitrogen with a residual oxygenconcentration less than or equal to 10 ppm.

In a second set of evaluations carried out using the structure of FIG.11, the presence/absence of two types of soldering defects (solder gapsand short circuits (solder bridges)) was scored on electronic boardssoldered in the machine. The evaluation was carried out both undernitrogen (delivery rate given above) and under air.

The observed results can be summarized as follows:

a1) residual oxygen level (by volume):

in the case of FIG. 9: in the absence of boards: ≅120,000 ppm; in thepresence of boards: <1%; in the case of FIG. 10: in the absence ofboards: ≅30,000 ppm; in the presence of boards: ≅70 ppm; in the case ofFIG. I 1: in the absence of boards: ≅8,000 ppm; in the presence ofboards: ≅45 ppm;

a2) dross evaluation:

The proportion of dross in the presence of boards was greatly decreasedfor all the tested configurations, and the last two configurations evenpermitted the flat surface of the wave to be preserved in itstraditional mirror appearance.

b) rate of soldering defects:

The results described here (in ppm defects) represent the cumulativeresults for 200 soldered boards.

under air under N₂ short circuit ≅4800 ≅2000 gaps in solder  ≅200  ≅70

Thus, read as a whole these results demonstrate that the wave solderingmachine and method according to the invention achieve, in dramaticfashion and with an extremely moderate nitrogen consumption, very lowresidual oxygen levels at the surface of the laminar wave, whichconsistently correspond with a clear improvement in the obtainedsoldering performance and with excellent results in terms of drossformation.

What is claimed is:
 1. A method for wave soldering or tinning comprisingthe steps of bringing a piece to be soldered or tinned into contact withat least one wave of liquid solder having a laminar form and including aflat surface and directing a protective gas over at least a portion ofthe wave by means of an injector that is located in a position adjacentto and downstream from the wave and is provided with a wall facing thewave, wherein said wall has at least a first group of openingspositioned thereon so as to generate a first gas jet directed toward theflat surface of the laminar wave, the gas emerging from the openingshaving a velocity between 0.5 and 30 m/s, and controlling flow rate ofoverflow of the solder wave in a downstream direction with a weir systemhaving an adjustable height with respect to the wave, wherein said weirsystem comprises a chute including an interior that dips into the solderreservoir, a skirt including an interior that dips into the solderreservoir or a plate that dips into the solder reservoir, wherein saidwall of the injector comprises a second group of openings positioned onthe wall so as to inject a second gas jet into the interior of thedipping chute or skirt, or between the plate and the injector.
 2. Themethod according to claim 1, wherein the velocity of gas emerging fromthe openings is between 0.5 and 10 m/s.
 3. The method according to claim2, wherein the velocity of gas emerging from the openings is between 0.5and 5 m/s.